1. Field of the Invention
The present invention relates to an aluminum-alloy clad sheet having high strength and excellent erosion resistance, which is used for an aluminum-alloy heat exchanger (hereinafter, aluminum may be referred to as Al).
2. Description of Related Art
To reduce weight of a motor vehicle body, aluminum alloy materials are increasingly used for the components of the motor vehicle, such as a heat exchanger component, in place of typically used copper alloy materials. An anti-corrosive aluminum alloy material formed of a multilayered clad sheet (sometimes referred to as clad material) is used for the aluminum alloy materials for a heat exchanger component. The clad sheet includes at least a core aluminum alloy sheet and an aluminum-alloy sacrificial anti-corrosive material cladded with each other. The clad sheet is a raw material for a heat exchanger, which is formed into a heat exchanger by brazing. Hence, a simply called aluminum clad sheet (or clad sheet) refers to an aluminum-alloy clad sheet (simply referred to as clad sheet in some case) before being brazed or being subjected to heating corresponding to brazing.
In the case where the clad sheet is assembled into a heat exchanger by brazing, the clad sheet is configured as a clad sheet (brazing sheet) including a core (sheet) of which one surface is cladded with an aluminum-alloy sacrificial anti-corrosive material (sheet), and the other surface is cladded with an aluminum-alloy brazing material (sheet).
FIG. 3 illustrates an exemplary aluminum-alloy heat exchanger (radiator) for a motor vehicle. As illustrated in FIG. 3, a radiator 100 generally has a configuration where aluminum-alloy radiating fins 112, each being formed into a corrugated shape, are integrally provided between a plurality of flat aluminum alloy tubes 111, and the respective ends of the tube 111 are opened to a header 113 and an undepicted tank. In the radiator 100 having such a configuration, a hot coolant is fed from a space of one tank to a space of the other tank through the tube 111, during which the coolant is cooled through heat exchange at the sites of the tube 111 and the radiating fins 112, and the cooled coolant is recirculated.
The tube 111 including the aluminum alloy material is formed of an aluminum-alloy brazing sheet 101 the section of which is illustrated in FIG. 4. In FIG. 4, the brazing sheet 101 includes an aluminum-alloy core 102 of which one side is cladded with an aluminum-alloy sacrificial anode material (sometimes referred to as skin material) 103, and the other side is cladded with an aluminum-alloy brazing material 104. In the case of an aluminum-alloy clad sheet illustrated in FIG. 4, only an aluminum-alloy sacrificial anti-corrosive material 103 is laminated on one surface of an aluminum-alloy core 102.
Such brazing sheet 101 is formed into a flat tube shape by, for example, forming rolls. The shaped brazing sheet 101 is then brazed by itself through resistance welding or heating for brazing into a fluid path formed of the tube 111 illustrated in FIG. 3.
The coolant used in the radiator includes a water-soluble medium as a main component to which a commercially available corrosion inhibitor and/or other agents are appropriately added. If the corrosion inhibitor is aged, however, acid is formed and may corrode the aluminum alloy materials such as the sacrificial material and the core. Hence, an aluminum alloy material having high corrosion resistance against the water-soluble medium must be used.
Hence, Al—Mn series (JIS 3000 series) alloy such as 3003 alloy having a composition of, for example, Al-0.15 mass % Cu-1.1 mass % Mn, which is specified in JIS H4000 is used as the aluminum alloy used for the brazing sheet or the clad sheet in light of corrosion resistance and strength. In addition, Al—Zn series alloy such as 7072 alloy having a composition of, for example, Al-1 mass % Zn or Al—Zn—Mg series (JIS 7000 series) alloy is used for the skin material 103, which is normally in contact with the coolant, in order to secure corrosion resistance and increase strength through diffusion of Mg into the core 102. Furthermore, Al—Si series (JIS 4000 series) alloy having a low melting point, such as 4045 alloy having a composition of, for example, Al-10 mass % Si, is used for the brazing material 104.
The radiator 100 is integrally assembled from the tubes 111 formed of such a brazing sheet 101, the corrugated radiating fins 112, and other components by brazing. The brazing technique includes flux brazing and Nocolok brazing using a noncorrosive flux, which are each performed at high temperature of about 600° C.
The above-described liquid coolant, which ranges from high temperature to low temperature and from high pressure to normal pressure, is normally distributed and circulated within the radiator 100, particularly the tube 111, assembled in the above way. Specifically, the tube 111 receives repeated stress for a long time due to such repeated variations in internal pressure, vibration of the motor vehicle itself, and/or other factors; hence, the tube 111 must have a sufficient strength to withstand such stress. If the tube 111 has a low strength, a fatigue failure may occur, which leads to a crack that develops in the tube 111. If the crack penetrates the tube 111, liquid leakage from the radiator is caused. Hence, improvement in strength of the radiator tube is an important issue.
Various proposals have been made to improve the strength or the fatigue characteristics of the radiator tube. In a typical proposal, average grain diameter of the core of the aluminum-alloy brazing sheet is controlled to improve resistance to the fatigue failure due to repeated bending of the tube 111, i.e., resistance to vibration fatigue under vibration of a motor vehicle. In another proposal, specific precipitates (intermetallic compounds) are distributed in a region of the core close to the boundary between the core and the sacrificial material of the brazed brazing sheet, in order to increase the strength of the region and improve the fatigue characteristics thereby.
In each of the proposals, however, the core of the radiator tube of the motor vehicle has a relatively large thickness, which is far above 0.20 mm. On the other hand, weight of the radiator is reduced for weight reduction of the motor vehicle to improve fuel efficiency in light of the global environmental issues. Thus, investigation is being made on further reduction in thickness of the radiator tube, namely, thickness of the aluminum-alloy brazing sheet.
In the case where the core of the radiator tube has a relatively large thickness as described above, the tube itself has a relatively high stiffness. In contrast, if thickness of the radiator tube, mainly thickness of the clad sheet such as brazing sheet is reduced, the tube itself has a relatively low stiffness. In addition, the coolant to be used is increasingly set to high pressure compared with in the past. The synergic effect among them results in an increase in sensitivity to the fatigue failure caused by the repeated stress in the clad sheet such as the brazing sheet having a reduced thickness. As a result, the clad sheet is likely to be degraded in fatigue characteristics. Such a fatigue failure causes a crack in the radiator tube. Such a crack is likely to penetrate the tube having a reduced thickness, which leads to liquid leakage from the radiator, resulting in a serious damage of the radiator.
Moreover, if the thickness of the clad sheet such as the brazing sheet is reduced, an erosion phenomenon may occur, i.e., the brazing material of the Al alloy brazing sheet erodes the core, leading to a reduction in thickness of the core. This also seriously damages the radiator.
Various ideas have been proposed on improvement in strength, fatigue characteristics, and/or erosion resistance of the radiator tube having such a reduced thickness. Such ideas typically include control of fine precipitates (intermetallic compounds). For example, in JP-A-8-246117, while the clad sheet has a thickness of about 0.25 mm, the core contains intermetallic compounds each having a size of about 0.02 to 0.2 μm in the number density of 10 to 2000 per cubic micrometer. The intermetallic compounds serve to improve the strength of the clad sheet through dispersion hardening, and to make recrystallized grains, which are formed in the clad sheet during heating for brazing, into a coarse pancake shape. In addition, diffusion elements are trapped at boundaries of the grains, which prevents variation of the composition of the core due to diffusion of the elements during brazing.
In JP-A-2002-126894, the average grain diameter of the core is reduced to 50 μm or less, and the number of compounds containing Al and Mn 0.01 to 0.1 μm (10 to 100 nm) in diameter, which are observed by a transmission electron microscope (TEM) at 60,000 power, is controlled to improve the erosion resistance.
In JP-A-2009-191293, a core aluminum alloy sheet having a small thickness of less than 0.25 mm has a structure where precipitates within a range of an average circle-equivalent diameter of 0.1 to 0.5 μm (100 to 500 nm) have an average number density of 150 per cubic micrometer or less, the precipitates being observed by TEM at 50,000 power on a rolling plane in the central part in the thickness of the core aluminum alloy sheet. This is based on the following finding described in JP-A-2009-191293. That is, the brazing sheet reduced in thickness has fatigue characteristics having two mechanisms of the fatigue failure, i.e., a first mechanism where propagation (speed) of a crack caused by a fatigue failure is dominant rather than initiation of the crack, and a second mechanism where initiation of a crack caused by a fatigue failure is dominant rather than propagation (speed) of the crack.
Specifically, different approaches, which are metallurgically effective for improving the fatigue characteristics, are taken for the two mechanisms of the fatigue failure. In the case where propagation (speed) of a crack caused by a fatigue failure is dominant rather than initiation of the crack, the propagation (speed) of the fatigue failure is greatly affected by the structure of the core aluminum alloy sheet of the clad sheet configuring a heat exchanger, i.e., affected by the average grain size and the average number density of relatively fine precipitates. In contrast, in the case where initiation of a crack caused by a fatigue failure is dominant rather than propagation (speed) of the crack, probability of the initiation of the crack is greatly affected by the structure of the core aluminum alloy sheet of the clad sheet configuring a heat exchanger, i.e., affected by the average grain size and the average number density of relatively coarse dispersed particles.
In particular, the approach described in JP-A-2009-191293 improves the fatigue characteristics in the case where propagation (speed) of a crack caused by a fatigue failure is dominant rather than initiation of the crack, which controls the average grain size and the average number density of relatively fine precipitates in the structure of the core aluminum alloy sheet as a raw material for the heat exchanger, or in the structure of the core aluminum alloy sheet subjected to heating equivalent to brazing in order to suppress the propagation of the fatigue failure so that the fatigue life (fatigue characteristics) is improved in the case where the propagation of the fatigue failure is dominant. In JP-A-2009-191293, the precipitates are assumed to be a general term of intermetallic compounds between alloy elements such as Si, Cu, Mn, and Ti or between inevitable elements such as Fe and Mg, and intermetallic compounds between the elements and Al, each of which can be identified based on the size through structure observation regardless of the constituent elements (composition).